The man who is for many the last word on books has been told his first word was “book.”

Ken Gloss, whose parents bought the storied Brattle Book Shop in 1949, the year before he was born, grew up helping out in the family business, surrounded by stacks, a kid in a candy store for the mind.

“Every day it’s like being Jim Hawkins in Treasure Island” off on another adventure on the hunt for something special, he said.

Rebecca Hyman

The man who is for many the last word on books has been told his first word was “book.”

Ken Gloss, whose parents bought the storied Brattle Book Shop in 1949, the year before he was born, grew up helping out in the family business, surrounded by stacks, a kid in a candy store for the mind.

“Every day it’s like being Jim Hawkins in Treasure Island” off on another adventure on the hunt for something special, he said.

Gloss, familiar to many as a frequent guest appraiser on PBS’ Antiques Roadshow, will give a free talk at the Bridgewater Public Library, “Is there value in your old and rare books?” followed by a question-and-answer session and free appraisals of all books and documents attendees bring with them on Tuesday, Oct. 8.

Books have an unmistakable allure and magic beyond their monetary value or beauty as objects, Gloss said.

“Books contain all of human knowledge,” he said.

So what makes some books treasures and others duds?

Like everything else, “It’s basically supply and demand,” Gloss said.

Sometimes, people assume an old book must be valuable, but, alas, that is not the case.

“You can have an old book that’s incredibly rare, but the customer for it might be even rarer,” Gloss said.

On the other hand, an old book may be desirable but far from rare and that can put a kibosh on the value. Take, for example, the Bible. It may be the greatest story ever told, but it was told a lot.

But, as usual, there are exceptions. Don’t, for example, discard that Gutenberg you might have lying around the house.

And even recent books can be valuable if they are highly sought after, a prime example being the first London edition of Harry Potter. Who knew it would become one of the best-selling books of all time? — Not the publisher, who issued a relatively small edition.

Gloss said appraisal nights are a lot of fun because he gets to meet some wonderful people and never knows what he’ll find. At least a couple of attendees usually bring something “really good,” with a value reaching into the hundreds or even thousands of dollars.

And then there are those rare occasions when someone has something truly spectacular, valuable enough to change a person’s life — those are thrilling moments for Gloss and the owner, of course.

Take, for example, the woman who walked into an appraisal night with a stack of papers that had been gathering dust, likely in the attic of her 200-year-old home. Imagine Gloss’ surprise when he found among the unpromising pile an extremely early edition of the Declaration of Independence worth three-quarters of a million dollars.

It was an unassuming document, a simple printed version on plain white paper, no signatures. Clearly, it had never caught the owner’s eye if she’d ever even gone through the whole stack.

But few printed words in history have had such a far-reaching effect. To look at that piece of paper is to picture the framers hammering out those unforgettable phrases, to hear the crunch of the antique printing press and the hoof beats on cobblestones as copies were distributed on horseback throughout the colonies — and the price the document commands is commensurate with its role in history.

“There has to be something important about a book historically, scientifically, literarily or for some other reason that makes it sought after. Sometimes it’s the illustrations. Sometimes famous artists do original lithographs. Sometimes it’s a gorgeous artistic binding. Sometimes it’s the subject,” Gloss said.

And sometimes it’s an inscription — bringing new meaning to the phrase, “Don’t judge a book by its cover.”

Once a person brought Gloss a tattered copy of “The Catcher in the Rye.” Fortunately, Gloss knew enough to look inside, where he found buried treasure, a long inscription from J.D. Salinger. That “worthless” book was worth $30,000 to $40,000.

For some book hounds, the pleasure is in the reading, Gloss said. But for others, the thrill is in the acquisition and possession. In fact, many bibliophiles don’t dare damage their most valuable first editions by subjecting them to their intended use, Gloss said. That’s what reprints from the bargain bin are for.

“A lot of collecting is about the fun of the hunt, going after something and being able to have what no one else does. It’s about bragging rights and prestige,” Gloss said.

Serious collectors may stalk a coveted book for decades, all the while hoping the prized tome will be in the next box, at the next yard sale, in the next dusty attic or on the next shelf.

When they finally find the object of their desire, it can be a bit of a let down. They lose their sense of purpose. It’s like the morning after Christmas.

But, fortunately, anyone that passionate about books likely has many more elusive conquests to pursue, Gloss said.

The Brattle Book Shop, located at 9 West St., Boston, in Downtown Crossing, is America’s oldest continuously operated bookstore and one of the largest, with a quarter of a million tomes. In addition to three floors packed with books, the store boasts a large open-air lot, where its rolling carts can be found weather permitting below a large black-and-white mural of well-known authors painted by area artists.

Ken Gloss will be appearing at the Bridgewater Public Library, 15 South St., in the Flora T. Little Meeting Room on Tuesday, Oct. 8 at 7 p.m. The event is free and open to the public.